The present invention relates to an image forming apparatus employing an electro-photographic method, and in particular, to the technology for transferring toner images carried on an image carrier to a recording material.
Concerning a transfer device for transferring a toner image on an image carrier, such as a photoconductor or an intermediate transfer body, widely used is a technology wherein while the recording material is brought into contact with the image carrier, an electro-charging is conducted onto the recording material from its rear surface using an opposite polarity to the charged electricity of the toner forming the images, whereby the toner images are transferred onto the recording material by electrostatic force.
In order to improve the contact of the recording material to the image carrier, a pressing means is proposed in which the recording material is pressed onto the image carrier (see Patent Documents 1 and 2).
[Patent Document 1] U.S. Pat. No. 4,947,214
[Patent Document 2] U.S. Pat. No. 5,923,921
The transfer device described in Patent Documents 1 and 2 makes it possible to evenly contact the total surface of recording material onto the image carrier, and to prevent any clearance between the image carrier and the transfer material, and thereby the transfer device can evenly transfer images and can form images of high quality. However the inventor of the present invention found the problem of the transfer device in Patent Documents 1 and 2, which will be explained referring to FIGS. 1 and 2.
The pressing means, being elastic plate X made of a PET (polyethylene terephthalate) film, presses recording sheet P employing straight edge X1 positioned perpendicularly to feeding direction V of recording material P, and thereby the pressing means presses recording sheet P onto image carrier Z. In this case, the width of straight edge X1 is nearly equal to the width of recording material P, as shown in FIG. 1(a).
However, recording material P does not always run precisely on the specific conveyance path, but occasionally meanders. Accordingly the edge of elastic plate X cannot directly contact image carrier Z, which causes deterioration of the surface of the image carrier, resulting in lowered image quality.
In order to overcome this problem, the inventor tried to shorten width W1 of straight edge X1 less than width W2 of recording material P, shown in FIG. 1(b).
However, the inventor found unevenness of images of a white clearness, which is also known as toner repelling, at areas U (which are hatched areas) of recording material P which were not pressed by elastic plate X.
This phenomenon will be explained below.
Referring to FIG. 2, in order to improve contact between recording material P and image carrier Z, the edge of elastic plate X is positioned at the position nearest to the transfer area, where is within the discharging area of transfer device TR.
Accordingly, recording material is influenced by elastic plate X. However, since elastic material X does not contact area U, shown in FIG. 1(b), it cannot affect those.
Primarily, the transfer condition is set to be optimum at the image transfer area, that is, the transfer condition is set to be optimum considering the influence of elastic material X. Therefore at areas U, being the edges of the image, unstable transfer is performed under less than the optimal conditions, resulting in images with white clearness, which is due to toner having been repelled.